Summary This project is directed towards uncovering the role that UL144, an orthologue of the herpesvirus entry mediator (HVEM, a TNFR superfamily member) encoded by virulent strains of human cytomegalovirus (HCMV, a b-herpesvirus), plays in modulating the host immune response. HCMV causes serious clinical complications in immunoncompromised individuals, and disease is associated with the loss of anti-HCMV T cell immunity. HVEM binds to many ligands, including the B and T lymphocyte attenuator (BTLA), a negative cosignaling receptor of the Ig-family that inhibits T-cell proliferation. Importantly, the HVEM-BTLA interaction serves as the first known example of a TNFR binding to a non TNF-related ligand. Until recently UL144 was an orphan receptor, but we have shown UL144 binds BTLA and can inhibit activation of BTLA-expressing T-cells. UL144 primary sequence is highly variable between clinical isolates of HCMV, segregating into three related groups, but all UL144 variants retain the ability to bind BTLA. This proposal aims to characterize the structure/function relationship of the UL144-BTLA interaction using both generated mutants and natural variants of UL144. Additionally, the relative ability of UL144 variants to modulate the activation/function of BTLA-expressing cells will be examined. Finally, the potential of UL144 to modulate BTLA-dependent events will be examined in various HCMV infected cell types using both wild-type and UL144-deficient viruses. This work will shed light upon whether UL144 targeting of the HVEM-BTLA cosignaling network contributes to HCMV modulation of the host anti-viral immune response, perhaps facilitating the establishment of lifelong infection.